Reduced balance and balance confidence are common impairments among individuals with lower-limb amputations. It is therefore important to develop and evaluate therapeutic interventions designed to improve balance and balance confidence in this population. Conventional approaches to balance training currently involve weight-shifting and stepping exercises that challenge single-limb stance and balance recovery. During this time, new prosthesis users typically wear a flexible prosthetic foot-ankle mechanism designed for walking. Although individuals with lower-limb amputations eventually adapt to the sudden balance requirements of their new prosthesis, their confidence is likely influenced by a persistent threat of falling. The long-term goal of this work is to compare a conventional approach of balance training with a modified program in which the balance requirements of the prosthesis user are gradually increased. To vary the balance requirements of the prosthesis user, a novel foot-ankle device will be developed at the Minneapolis VA Health Care System (MVAHCS), which will allow therapists to gradually adjust the stability of the foot-ankle system as patients learn to safely transfer weight over their prosthesis. This new device will enable physical therapists to maximize stability at the beginning of physical therapy, thereby allowing patients to weight shift with a highly stable system. As the patient's confidence increases, therapists will be able to gradually decrease the stability of the device. The primary goals of our development work will be to develop a prosthetic foot-ankle system that is durable and that allows for easy adjustments (i.e., a system that requires little extra training or tools to accomplish). Consideration will also be given to a method of tracking these adjustments so that therapists can document the patient's progress during the course of physical therapy. Following each design iteration, we will mechanically test the foot-ankle system for static proof, ultimate strength, and cyclic fatigue according to the ISO 10328 standard for lower-limb prosthetic components. We will then perform human subject testing to assess the standing and walking performance of the device and obtain feedback to guide design modifications. Using three-dimensional motion analysis and a clinical balance assessment system, we will collect kinematic, kinetic, and postural stability data on a group of experienced prosthesis users (n=8) while they stand and walk with the proposed foot-ankle system. The goal of this testing will be to ensure biomimetic function of the ankle and verify expected kinematic changes with regard to its range-of-motion settings. We will also conduct human subject testing on a group of new amputees (n=4) undergoing physical therapy at the MVAHCS, which will provide an opportunity for the prosthetists and physical therapists involved in the study to gain experience with the device. In parallel with our development efforts, we will develop a physical therapy protocol that leverages the unique features of the foot-ankle system. The goal of this effort will be to provide an integrated solution to balance training for Veterans with lower-limb amputations. Finally, we will implement an appropriately scoped quality management system to document design criteria, reduce overall development costs, and increase the likelihood of technology transfer. Ultimately, we believe that the foot-ankle system developed in this project will lead to new approaches of physical therapy for individuals with lower-limb amputations, improving mobility, participation in social activities, and quality of life for this population.